Titanium dioxide is a common and widely studied photocatalyst due to its appealing attributes such as non-toxicity, chemical inertness and high photocatalytic activity. Since the first report of photocatalytic purification of water using titania in 1977, use of TiO2 has been widely investigated in literature. The large band gap of TiO2 (˜3.2 eV) permits it to absorb photons in the UV region, which results in production of electron-hole pairs that migrate to the catalyst surface and participate in redox reactions with organic species. Upon doping with nitrogen or metal ions, TiO2 shows photocatalytic activity under visible radiation, which increases its commercial viability manifold.
In recent years, there has been increased interest in the use of nanosized titania powders due to enhancements in photocatalytic activity. This enhancement results from changes in properties such as crystallinity, surface area for reaction, and density of surface groups like OH that accompany the fine size. Because separation of suspended fine particles from water has been a major obstacle, use of nanoparticles of titania in applications such as waste-water treatment have been limited. Strategies that have been investigated to address this obstacle include immobilization of titania particle onto planar substrates or reactor walls. However, the reduction in available surface area of the catalyst and the transport limitations to the surface can lead to diminished photocatalytic activity, thereby limiting the usefulness of these strategies.
Another approach has involved the synthesis of polymer-titania microcomposites. In one report, supercritical fluids were used to incorporate insoluble inorganic nanoparticles into organic networks (see B. Yue; Y. Wang; C. Y. Huang; R. Pfeffer; Z. Iqbal, Journal of Nanoscience and Nanotechnology 2007, 7, (3), 994-1000, and Y. Wang; Z.-H. Jiang; F.-J. Yang, Materials Science & Engineering, B: Solid-State Materials for Advanced Technology 2006, 128, (1-3), 229-233; which are incorporated herein by reference). However, the ensuing aggregation of the nanoparticles reduced the effectiveness of the microcomposite (see K. Matsuyama; K. Mishima; K. I. Hayashi; H. Matsuyama, Journal of Nanoparticle Research 2003, 5, (1-2), 87-95, also incorporated herein).